This invention relates to sulfur silane coupling agents which are latent, that is, they are in a state of reduced activity until such a time as one finds it useful to activate them. The invention also relates to the manufacture of rubbers including inorganic fillers and these silane coupling agents, as well as to the manufacture of the silanes.
The majority of art in the use of sulfur-containing coupling agents in rubber involves silanes containing one or more of the following chemical bond types:
Sxe2x80x94H (mercapto), Sxe2x80x94S (disulfide or polysulfide), or Cxe2x95x90S (thiocarbonyl). Mercaptosilanes have offered superior coupling at substantially reduced loadings; however, their high chemical reactivity with organic polymers leads to unacceptably high viscosities during processing and premature curing (scorch). Their undesirability is aggravated by their odor. As a result, other, less reactive coupling agents have been found. Hence, a compromise must be found between coupling and the associated final properties, processability, and required loading levels, which invariably leads to the need to use substantially higher coupling agent loadings than would be required with mercaptosilanes, and often also to the need to deal with less than optimal processing conditions, both of which lead to higher costs.
The prior art discloses acylthioalkyl silanes, such as CH3C(xe2x95x90O)S(CH2)1-3Si(OR)3 (M. G. Voronkov et al. in Inst. Org. Khim., Irkutsk, Russia) and HOC(xe2x95x90O)CH2CH2C(xe2x95x90O)S(CH2)3Si(OC2H5)3 (U.S. Pat. No. 3,922,436 to R. Bell et al.). Takeshita and Sugawara disclosed in Japanese Patent JP 63270751 A2 the use of compounds represented by the general formula CH2xe2x95x90C(CH3)C(xe2x95x90O)S(CH2)1-6Si(OCH3)3 in tire tread compositions; but these compounds are not desirable because the unsaturation xcex1,xcex2 to the carbonyl group of the thioester has the undesirable potential to polymerize during the compounding process or during storage.
Prior art by Yves Bomal and Olivier Durel in Australian Patent AU-A-10082/97 discloses the use in rubber of silanes of the structure represented by R1nX3xe2x88x92nSixe2x80x94(Alk)m(Ar)pxe2x80x94S(Cxe2x95x90O)xe2x80x94R where R1 is phenyl or alkyl; X is halogen, alkoxy, cycloalkoxy, acyloxy, or OH; Alk is alkyl; Ar is aryl; R is alkyl, alkenyl, or aryl; n is 0 to 2; and m and p are each 0 or 1, but not both zero. This prior art, however, stipulates that compositions of the structures of Formula (1P) must be used in conjunction with functionalized siloxanes. In addition, the prior art does not disclose or suggest the use of compounds of Formula (1P) as latent mercaptosilane coupling agents, nor does it disclose or suggest the use of these compounds in any way which would give rise to the advantages of using them as a source of latent mercaptosilane.
U.S. Pat. No. 4,519,430 to Ahmad et al. and U.S. Pat. No. 4,184,998 to Shippy et al. disclose the blocking of a mercaptosilane with an isocyanate to form a solid which is added to a tire composition, which mercaptan reacts into the tire during heating, which could happen at any time during processing since this is a thermal mechanism. The purpose of this silane is to avoid the sulfur smell of the mercaptosilane, not to improve the processing of the tire. Moreover, the isocyanate used has toxicity issues when used to make the silane and when released during rubber processing.
U.S. Pat. No. 3,957,718 to Porchet et al. discloses compositions containing silica, phenoplasts or aminoplasts, and silanes, such as xanthates, thioxanthates, and dithiocarbamates; however, the prior art does not disclose or suggest the use of these silanes as latent mercaptosilane coupling agents, nor does it suggest or disclose the advantage of using them as a source of latent mercaptosilane. There remains a need for effective latent coupling agents which exhibit the advantages of mercaptosilanes without exhibiting the disadvantages such as described herein.
The silanes of the present invention are mercaptosilane derivatives in which the mercapto group is blocked (xe2x80x9cblocked mercaptosilanesxe2x80x9d), i.e., the mercapto hydrogen atom is replaced by another group (hereafter referred to as xe2x80x9cblocking groupxe2x80x9d). Specifically, the silanes of the present invention are blocked mercaptosilanes in which the blocking group contains an unsaturated heteroatom or carbon chemically bound directly to sulfur via a single bond. This blocking group optionally may be substituted with one or more carboxylate ester or carboxylic acid functional groups. The use of these silanes in the manufacture of inorganic filled rubbers is taught wherein they are deblocked by the use of a deblocking agent during the manufacturing process. The manufacture of such silanes is taught as well.
Silane Structures
The blocked mercaptosilanes can be represented by the Formulae (1-2):
[[(ROC(xe2x95x90O))pxe2x80x94(G)j]kxe2x80x94Yxe2x80x94S]rxe2x80x94Gxe2x80x94(SiX3)sxe2x80x83xe2x80x83(1); 
and
[(X3Si)qxe2x80x94G]axe2x80x94[Yxe2x80x94[Sxe2x80x94Gxe2x80x94SiX3]b]cxe2x80x83xe2x80x83(2) 
wherein
Y is a polyvalent species (Q)zA(xe2x95x90E), preferably selected from the group consisting of xe2x80x94C(xe2x95x90NR)xe2x80x94; xe2x80x94SC(xe2x95x90NR)xe2x80x94; xe2x80x94SC(xe2x95x90O)xe2x80x94; (xe2x80x94NR)C(xe2x95x90O)xe2x80x94; (xe2x80x94NR)C(xe2x95x90S)xe2x80x94; xe2x80x94OC(xe2x95x90O)xe2x80x94; xe2x80x94OC(xe2x95x90S)xe2x80x94; xe2x80x94C(xe2x95x90O)xe2x80x94; xe2x80x94SC(xe2x95x90S)xe2x80x94; xe2x80x94C(xe2x95x90S)xe2x80x94; xe2x80x94S(xe2x95x90O)xe2x80x94; xe2x80x94S(xe2x95x90O)2xe2x80x94; xe2x80x94OS(xe2x95x90O)2xe2x80x94; (xe2x80x94NR)S(xe2x95x90O)2xe2x80x94; xe2x80x94SS(xe2x95x90O)xe2x80x94; xe2x80x94OS(xe2x95x90O)xe2x80x94; (xe2x80x94NR)S(xe2x95x90O)xe2x80x94; xe2x80x94SS(xe2x95x90O)2xe2x80x94; (xe2x80x94S)2P(xe2x95x90O)xe2x80x94; xe2x80x94(xe2x80x94S)P(xe2x95x90O)xe2x80x94; xe2x80x94P(xe2x95x90O)(xe2x80x94)2; (xe2x80x94S)2P(xe2x95x90S)xe2x80x94; xe2x80x94(xe2x80x94S)P(xe2x95x90S)xe2x80x94; xe2x80x94P(xe2x95x90S)(xe2x80x94)2; (xe2x80x94NR)2P(xe2x95x90O)xe2x80x94; (xe2x80x94NR)(xe2x80x94S)P(xe2x95x90O)xe2x80x94; (xe2x80x94O)(xe2x80x94NR)P(xe2x80x94O)xe2x80x94; (xe2x80x94O)(xe2x80x94S)P(xe2x95x90O)xe2x80x94; (xe2x80x94O)2P(xe2x95x90O)xe2x80x94; xe2x80x94(xe2x80x94O)P(xe2x95x90O)xe2x80x94; xe2x80x94(xe2x80x94NR)P(xe2x95x90O)xe2x80x94; (xe2x80x94NR)2P(xe2x95x90S)xe2x80x94; (xe2x80x94NR)(xe2x80x94S)P(xe2x95x90S)xe2x80x94; (xe2x80x94O)(xe2x80x94NR)P(xe2x95x90S)xe2x80x94; (xe2x80x94O)(xe2x80x94S)P(xe2x95x90S)xe2x80x94; (xe2x80x94O)2P(xe2x95x90S)xe2x80x94; xe2x80x94(xe2x80x94O)P(xe2x95x90S)xe2x80x94; and xe2x80x94(xe2x80x94NR)P(xe2x95x90S)xe2x80x94; each wherein the atom (A) attached to the unsaturated heteroatom (E) is attached to the sulfur, which in turn is linked via a group G to the silicon atom;
each R is chosen independently from hydrogen, straight, cyclic, or branched alkyl that may or may not contain unsaturation, alkenyl groups, aryl groups, and aralkyl groups, with each R containing from 1 to 18 carbon atoms;
each G is independently a monovalent or polyvalent group derived by substitution of alkyl, alkenyl, aryl, or aralkyl wherein G can contain from 1 to 18 carbon atoms, with the proviso that if Y is xe2x80x94C(xe2x95x90O)xe2x80x94, G is not such that the blocked mercaptosilane would contain an xcex1,xcex2-unsaturated carbonyl, and if G directly bonded to Y is univalent (i.e., if p=0), G can be a hydrogen atom;
X is independently selected from the group consisting of xe2x80x94Cl, xe2x80x94Br, ROxe2x80x94, RC(xe2x95x90O)Oxe2x80x94, R2Cxe2x95x90NOxe2x80x94, R2NOxe2x80x94 or R2Nxe2x80x94, xe2x80x94R, xe2x80x94(OSiR2)t(OSiR3) wherein each R and G is as above and at least one X is not xe2x80x94R;
Q is oxygen, sulfur, or (xe2x80x94NRxe2x80x94);
A is carbon, sulfur, phosphorus, or sulfonyl;
E is oxygen, sulfur, or NR;
p is 0 to 5; r is 1 to 3; z is 0 to 2; q is 0 to 6; a is 0 to 7; b is 1 to 3; j is 0 to 1, but it may be 0 only if p is 1; c is 1 to 6, preferably 1 to 4; t is 0 to 5; s is 1 to 3; k is 1 to 2; with the provisos that (A) if A is carbon, sulfur, or sulfonyl, then (i) a+b=2 and (ii) k=1; (B) if A is phosphorus, then a+b=3 unless both (i) c greater than 1 and (ii) b=1, in which case a=c+1; and (C) if A is phosphorus, then k is 2.
As used herein, xe2x80x9calkylxe2x80x9d includes straight, branched, and cyclic alkyl groups, and xe2x80x9calkenylxe2x80x9d includes straight, branched, and cyclic alkenyl groups containing one or more carbonxe2x80x94carbon double bonds. Specific alkyls include methyl, ethyl, propyl, isobutyl, and specific aralkyls include phenyl, tolyl, and phenethyl. As used herein, xe2x80x9ccyclic alkylxe2x80x9d or xe2x80x9ccyclic alkenylxe2x80x9d also includes bicyclic and higher cyclic structures, as well as cyclic structures further substituted with alkyl groups. Representative examples include norbornyl, norbornenyl, ethylnorbornyl, ethylnorbornenyl, ethylcyclohexyl, ethylcyclohexenyl, and cyclohexylcyclohexyl.
Representative examples of the functional groups (xe2x80x94YSxe2x80x94) present in the silanes of the present invention include thiocarboxylate ester, xe2x80x94C(xe2x95x90O)Sxe2x80x94 (any silane with this functional group is a xe2x80x9cthiocarboxylate ester silanexe2x80x9d); dithiocarboxylate, xe2x80x94C(xe2x95x90S)Sxe2x80x94 (any silane with this functional group is a xe2x80x9cdithiocarboxylate ester silanexe2x80x9d); thiocarbonate ester, xe2x80x94Oxe2x80x94C(xe2x95x90O)Sxe2x80x94 (any silane with this functional group is a xe2x80x9cthiocarbonate ester silanexe2x80x9d); dithiocarbonate ester, xe2x80x94Sxe2x80x94C(xe2x95x90O)Sxe2x80x94 and xe2x80x94Oxe2x80x94C(xe2x95x90S)Sxe2x80x94 (any silane with this functional groups is a xe2x80x9cdithiocarbonate ester silanexe2x80x9d); trithiocarbonate ester, xe2x80x94Sxe2x80x94C(xe2x95x90S)Sxe2x80x94 (any silane with this functional group is a xe2x80x9ctrithiocarbonate ester silanexe2x80x9d); dithiocarbamate ester, (xe2x80x94Nxe2x80x94)C(xe2x95x90S)Sxe2x80x94 (any silane with this functional group is a xe2x80x9cdithiocarbamate ester silanexe2x80x9d); thiosulfonate ester, xe2x80x94S(xe2x95x90O)2Sxe2x80x94 (any silane with this functional group is a xe2x80x9cthiosulfonate ester silanexe2x80x9d); thiosulfate ester, xe2x80x94Oxe2x80x94S(xe2x95x90O)2Sxe2x80x94 (any silane with this functional group is a xe2x80x9cthiosulfate ester silanexe2x80x9d); thiosulfamate ester, (xe2x80x94Nxe2x80x94)S(xe2x95x90O)2Sxe2x80x94 (any silane with this functional group is a xe2x80x9cthiosulfamate ester silanexe2x80x9d); thiosulfinate ester, xe2x80x94S(xe2x95x90O)Sxe2x80x94 (any silane with this functional group is a xe2x80x9cthiosulfinate ester silanexe2x80x9d); thiosulfite ester, xe2x80x94Oxe2x80x94S(xe2x95x90O)Sxe2x80x94 (any silane with this functional group is a xe2x80x9cthiosulfite ester silanexe2x80x9d); thiosulfimate ester, (xe2x80x94Nxe2x80x94)S(xe2x95x90O)xe2x80x94Sxe2x80x94 (any silane with this functional group is a xe2x80x9cthiosulfimate ester silanexe2x80x9d); thiophosphate ester, P(xe2x95x90O)(Oxe2x80x94)2(Sxe2x80x94) (any silane with this functional group is a xe2x80x9cthiophosphate ester silanexe2x80x9d); dithiophosphate ester, P(xe2x95x90O)(Oxe2x80x94)(Sxe2x80x94)2 or P(xe2x95x90S)(Oxe2x80x94)2(Sxe2x80x94) (any silane with this functional group is a xe2x80x9cdithiophosphate ester silanexe2x80x9d); trithiophosphate ester, P(xe2x95x90O)(Sxe2x80x94)3 or P(xe2x95x90S)(Oxe2x80x94)(Sxe2x80x94)2 (any silane with this functional group is a xe2x80x9ctrithiophosphate ester silanexe2x80x9d); tetrathiophosphate ester P(xe2x95x90S)(Sxe2x80x94)3 (any silane with this functional group is a xe2x80x9ctetrathiophosphate ester silanexe2x80x9d); thiophosphamate ester, xe2x80x94P(xe2x95x90O)(xe2x80x94Nxe2x80x94)(Sxe2x80x94) (any silane with this functional group is a xe2x80x9cthiophosphamate ester silanexe2x80x9d); dithiophosphamate ester, xe2x80x94P(xe2x95x90S)(xe2x80x94Nxe2x80x94)(Sxe2x80x94) (any silane with this functional group is a xe2x80x9cdithiophosphamate ester silanexe2x80x9d); thiophosphoramidate ester, (xe2x80x94Nxe2x80x94)P(xe2x95x90O)(Oxe2x80x94)(Sxe2x80x94) (any silane with this functional group is a xe2x80x9cthiophosphoramidate ester silanexe2x80x9d); dithiophosphoramidate ester, (xe2x80x94Nxe2x80x94)P(xe2x95x90O)(Sxe2x80x94)2 or (xe2x80x94Nxe2x80x94)P(xe2x95x90S)(Oxe2x80x94)(Sxe2x80x94) (any silane with this functional group is a xe2x80x9cdithiophosphoramidate ester silanexe2x80x9d); trithiophosphoramidate ester, (xe2x80x94Nxe2x80x94)P(xe2x95x90S)(Sxe2x80x94)2 (any silane with this functional group is a xe2x80x9ctrithiophosphorarnidate ester silanexe2x80x9d).
Novel silanes of the present invention are those wherein Y groups are xe2x80x94C(xe2x95x90NR)xe2x80x94; xe2x80x94SC(xe2x95x90NR)xe2x80x94; xe2x80x94SC(xe2x95x90O)xe2x80x94; xe2x80x94OC(xe2x95x90O)xe2x80x94; xe2x80x94S(xe2x95x90O)xe2x80x94; xe2x80x94S(xe2x95x90O)2xe2x80x94; xe2x80x94OS(xe2x95x90O)2xe2x80x94; xe2x80x94(NR)S(xe2x95x90O)2xe2x80x94; xe2x80x94SS(xe2x95x90O)xe2x80x94; xe2x80x94OS(xe2x95x90O)xe2x80x94; xe2x80x94(NR)S(xe2x95x90O)xe2x80x94; xe2x80x94SS(xe2x95x90O)2xe2x80x94; (xe2x80x94S)2P(xe2x95x90O)xe2x80x94; xe2x80x94(xe2x80x94S)P(xe2x95x90O)xe2x80x94; xe2x80x94P(xe2x95x90O)(xe2x80x94)2; (xe2x80x94S)2P(xe2x95x90S)xe2x80x94; xe2x80x94(xe2x80x94S)P(xe2x95x90S)xe2x80x94; xe2x80x94P(xe2x95x90S)(xe2x80x94)2; (xe2x80x94NR)2P(xe2x95x90O)xe2x80x94; (xe2x80x94NR)(xe2x80x94S)P(xe2x95x90O)xe2x80x94; (xe2x80x94O)(xe2x80x94NR)P(xe2x95x90O)xe2x80x94; (xe2x80x94O)(xe2x80x94S)P(xe2x95x90O)xe2x80x94; (xe2x80x94O)2P(xe2x95x90O)xe2x80x94; xe2x80x94(xe2x80x94O)P(xe2x95x90O)xe2x80x94; xe2x80x94(xe2x80x94NR)P(xe2x95x90O)xe2x80x94; (xe2x80x94NR)2P(xe2x95x90S)xe2x80x94; (xe2x80x94NR)(xe2x80x94S)P(xe2x95x90S)xe2x80x94; (xe2x80x94O)(xe2x80x94NR)P(xe2x95x90S)xe2x80x94; (xe2x80x94O)(xe2x80x94S)P(xe2x95x90S)xe2x80x94; (xe2x80x94O)2P(xe2x95x90S)xe2x80x94; xe2x80x94(xe2x80x94O)P(xe2x95x90S)xe2x80x94; and xe2x80x94(xe2x80x94NR)P(xe2x95x90S)xe2x80x94. Particularly preferred of these are xe2x80x94OC(xe2x95x90O)xe2x80x94; xe2x80x94SC(xe2x95x90O)xe2x80x94; xe2x80x94S(xe2x95x90O)xe2x80x94; xe2x80x94OS(xe2x95x90O)xe2x80x94; xe2x80x94(xe2x80x94S)P(xe2x95x90O)xe2x80x94; and xe2x80x94P(xe2x95x90O)(xe2x80x94)2.
Another novel silane would be one wherein Y is xe2x80x94C(xe2x95x90O)xe2x80x94 wherein G has a primary carbon attached to the carbonyl and is a C2xe2x80x94C2 alkyl, more preferably a C6-C8 alkyl.
Another preferred novel structure is of the form X3SiGSC(xe2x95x90O)GC(xe2x95x90O)SGSiX3 wherein G is a divalent hydrocarbon.
Examples of G include xe2x80x94(CH2)nxe2x80x94 wherein n is 1 to 12, diethylene cyclohexane, 1,2,4-triethylene cyclohexane, and diethylene benzene. It is preferred that the sum of the carbon atoms within the G groups within the molecule are from 3 to 18, more preferably 6 to 14. This amount of carbon in the blocked mercaptosilane facilitates the dispersion of the inorganic filler into the organic polymers, thereby improving the balance of properties in the cured filled rubber.
Preferable R groups are alkyls of C1 to C4 and H.
Specific examples of X are methoxy, ethoxy, isobutoxy, propoxy, isopropoxy, acetoxy, and oximato. Methoxy, acetoxy, and ethoxy are preferred. At least one X must be reactive (i.e., hydrolyzable).
Preferred embodiments are wherein p is 0 to 2; X is ROxe2x80x94 or RC(xe2x95x90O)Oxe2x80x94; R is hydrogen, phenyl, isopropyl, cyclohexyl, or isobutyl; G is a substituted phenyl or substituted straight chain alkyl of C2 to C12. The most preferred embodiments include those wherein p is zero, X is ethoxy, and G is a C3-C12 alkyl derivative.
Representative examples of the silanes of the present invention include:
2-triethoxysilyl-1-ethyl thioacetate; 2-trimethoxysilyl-1-ethyl thioacetate; 2-(methyldimethoxysilyl)-1-ethyl thioacetate; 3-trimethoxysilyl-1-propyl thioacetate; triethoxysilylmethyl thioacetate; trimethoxysilylmethyl thioacetate; triisopropoxysilylmethyl thioacetate; methyldiethoxysilylmethyl thioacetate; methyldimethoxysilylmethyl thioacetate; methyldiisopropoxysilylmethyl thioacetate; dimethylethoxysilylmethyl thioacetate; dimethylmethoxysilylmethyl thioacetate; dimethylisopropoxysilylmethyl thioacetate; 2-triisopropoxysilyl-1-ethyl thioacetate; 2-(methyldiethoxysilyl)-1-ethyl thioacetate; 2-(methyldiisopropoxysilyl)-1-ethyl thioacetate; 2-(dimethylethoxysilyl)-1-ethyl thioacetate; 2-(dimethylmethoxysilyl)-1-ethyl thioacetate; 2-(dimethylisopropoxysilyl)-1-ethyl thioacetate; 3-triethoxysilyl-1-propyl thioacetate; 3-triisopropoxysilyl-1-propyl thioacetate; 3-methyldiethoxysilyl-1-propyl thioacetate; 3-methyldimethoxysilyl-1-propyl thioacetate; 3-methyldiisopropoxysilyl-1-propyl thioacetate; 1-(2-triethoxysilyl-1-ethyl)-4-thioacetylcyclohexane; 1-(2-triethoxysilyl-1-ethyl)-3-thioacetylcyclohexane; 2-triethoxysilyl-5-thioacetylnorbornene; 2-triethoxysilyl-4-thioacetylnorbornene; 2-(2-triethoxysilyl-1-ethyl)-5-thioacetylnorbornene; 2-(2-triethoxysilyl-1-ethyl)-4-thioacetylnorbornene; 1-(1-oxo-2-thia-5-triethoxysilylpenyl)benzoic acid; 6-triethoxysilyl-1-hexyl thioacetate; 1-triethoxysilyl-5-hexyl thioacetate; 8-triethoxysilyl-1-octyl thioacetate; 1-triethoxysilyl-7-octyl thioacetate; 6-triethoxysilyl-1-hexyl thioacetate; 1-triethoxysilyl-5-octyl thioacetate; 8-trimethoxysilyl-1-octyl thioacetate; 1-trimethoxysilyl-7-octyl thioacetate; 10-triethoxysilyl-1-decyl thioacetate; 1-triethoxysilyl-9-decyl thioacetate; 1-triethoxysilyl-2-butyl thioacetate; 1-triethoxysilyl-3-butyl thioacetate; 1-triethoxysilyl-3-methyl-2-butyl thioacetate; 1-triethoxysilyl-3-methyl-3-butyl thioacetate; 3-trimethoxysilyl-1-propyl thiooctanoate; 3-triethoxysilyl-1-propyl thiopalmitate; 3-triethoxysilyl-1-propyl thiooctanoate; 3-triethoxysilyl-1-propyl thiobenzoate; 3-triethoxysilyl-1-propyl thio-2-ethylhexanoate; 3-methyldiacetoxysilyl-1-propyl thioacetate; 3-triacetoxysilyl-1-propyl thioacetate; 2-methyldiacetoxysilyl-1-ethyl thioacetate; 2-triacetoxysilyl-1-ethyl thioacetate; 1-methyldiacetoxysilyl-1-ethyl thioacetate; 1-triacetoxysilyl-1-ethyl thioacetate; tris-(3-triethoxysilyl-1-propyl)trithiophosphate; bis-(3-triethoxysilyl-1-propyl)methyldithiophosphonate; bis-(3-triethoxysilyl-1-propyl)ethyldithiophosphonate; 3-triethoxysilyl-1-propyldimethylthiophosphinate; 3-triethoxysilyl-1-propyldiethylthiophosphinate; tris-(3-triethoxysilyl-1-propyl)tetrathiophosphate; bis-(3-triethoxysilyl-1-propyl)methyltrithiophosphonate; bis-(3-triethoxysilyl-1-propyl)ethyltrithiophosphonate; 3-triethoxysilyl-1-propyldimethyldithiophosphinate; 3-triethoxysilyl-1-propyldiethyldithiophosphinate; tris-(3-methyldimethoxysilyl-1-propyl)trithiophosphate; bis-(3-methyldimethoxysilyl-1-propyl)methyldithiophosphonate; bis-(3-methyldimethoxysilyl-1-propyl)ethyldithiophosphonate; 3-methyldimethoxysilyl-1-propyldimethylthiophosphinate; 3-methyldimethoxysilyl-1-propyldiethylthiophosphinate; 3-triethoxysilyl-1-propylmethylthiosulphate; 3-triethoxysilyl-1-propylmethanethiosulphonate; 3-triethoxysilyl-1-propylethanethiosulphonate; 3-triethoxysilyl-1-propylbenzenethiosulphonate; 3-triethoxysilyl-1-propyltoluenethiosulphonate; 3-triethoxysilyl-1-propylnaphthalenethiosulphonate; 3-triethoxysilyl-1-propylxylenethiosulphonate; triethoxysilylmethylmethylthiosulphate; triethoxysilylmethylmethanethiosulphonate; triethoxysilylmethylethanethiosulphonate; triethoxysilylmethylbenzenethiosulphonate; triethoxysilylmethyltoluenethiosulphonate; triethoxysilylmethylnaphthalenethiosulphonate; and triethoxysilylmethylxylenethiosulphonate.
Mixtures of various blocked mercaptosilanes may be used, including wherein synthetic methods result in a distribution of various silanes or where mixes of blocked mercaptosilanes are used for their various blocking or leaving functionalities. Moreover, it is understood that the partial hydrolyzates of these blocked mercaptosilanes (i.e., blocked mercaptosiloxanes) may also be encompassed by the blocked mercaptosilanes herein, in that these partial hydrolyzates will be a side product of most methods of manufacture of the blocked mercaptosilane or can occur upon storage of the blocked mercaptosilane, especially in humid conditions.
The silane, if liquid, may be loaded on a carrier, such as a porous polymer, carbon black, or silica so that it is in solid form for delivery to the rubber. In a preferred mode, the carrier would be part of the inorganic filler to be used in the rubber.
Manufacture of Silanes
The methods of preparation for blocked mercaptosilanes can involve esterification of sulfur in a sulfur-containing silane and direct incorporation of the thioester group into a silane, either by substitution of an appropriate leaving group or by addition across a carbonxe2x80x94carbon double bond. Illustrative examples of synthetic procedures for the preparation of thioester silanes would include: Reaction 1) the reaction between a mercaptosilane and an acid anhydride corresponding to the thioester group present in the desired product; Reaction 2) reaction of an alkali metal salt of a mercaptosilane with the appropriate acid anhydride or acid halide; Reaction 3) the transesterification between a mercaptosilane and an ester, optionally using any appropriate catalyst such as an acid, base, tin compound, titanium compound, transition metal salt, or a salt of the acid corresponding to the ester; Reaction 4) the transesterification between a thioester silane and another ester, optionally using any appropriate catalyst such as an acid, base, tin compound, titanium compound, transition metal salt, or a salt of the acid corresponding to the ester; Reaction 5) the transesterification between a 1-sila-2-thiacyclopentane or a 1-sila-2-thiacyclohexane and an ester, optionally using any appropriate catalyst such as an acid, base, tin compound, titanium compound, transition metal salt, or a salt of the acid corresponding to the ester; Reaction 6) the free radical addition of a thioacid across a carbonxe2x80x94carbon double bond of an alkene-functional silane, catalyzed by UV light, heat, or the appropriate free radical initiator wherein, if the thioacid is a thiocarboxylic acid, the two reagents are brought into contact with each other in such a way as to ensure that whichever reagent is added to the other is reacted substantially before the addition proceeds; and Reaction 7) the reaction between an alkali metal salt of a thioacid with a haloalkylsilane.
Acid halides include but are not limited to, in addition to organic acid halides, inorganic acid halides, such as POT3, SOT2, SO2T2, COT2, CST2, PST3 and PT3, wherein T is a halide. Acid anhydrides include but are not limited to, in addition to organic acid anhydrides (and their sulfur analogs), inorganic acid anhydrides such as SO3, SO2, P2O5, P2S5, H2S2O7, CO2, COS, and CS2.
Illustrative examples of synthetic procedures for the preparation of thiocarboxylate-functional silanes would include: Reaction 8) the reaction between a mercaptosilane and a carboxylic acid anhydride corresponding to the thiocarboxylate group present in the desired product; Reaction 9) reaction of an alkali metal salt of a mercaptosilane with the appropriate carboxylic acid anhydride or acid halide; Reaction 10) the transesterification between a mercaptosilane and a carboxylate ester, optionally using any appropriate catalyst, such as an acid, base, tin compound, titanium compound, transition metal salt, or a salt of the acid corresponding to the carboxylate ester; Reaction 11) the transesterification between a thiocarboxylate-functional silane and another ester, optionally using any appropriate catalyst, such as an acid, base, tin compound, titanium compound, transition metal salt, or a salt of the acid corresponding to the other ester; Reaction 12) the transesterification between a 1-sila-2-thiacyclopentane or a 1-sila-2-thiacyclohexane and a carboxylate ester, optionally using any appropriate catalyst, such as an acid, base, tin compound, titanium compound, transition metal salt, or a salt of the acid corresponding to the carboxylate ester; Reaction 13) the free radical addition of a thiocarboxylic acid across a carbonxe2x80x94carbon double bond of an alkene-functional silane, catalyzed by UV light, heat, or the appropriate free radical initiator; and Reaction 14) the reaction between an alkali metal salt of a thiocarboxylic acid with a haloalkylsilane.
Reactions 1 and 8 can be carried out by distilling a mixture of the mercaptosilane and the acid anhydride and optionally a solvent. Appropriate boiling temperatures of the mixture are in the range of 60xc2x0 to 200xc2x0 C.; preferably 70xc2x0 to 170xc2x0 C.; optionally 50xc2x0 to 250xc2x0 C. This process leads to a chemical reaction in which the mercapto group of the mercaptosilane is esterified to the thioester silane analog with release of an equivalent of the corresponding acid. The acid typically is more volatile than the acid anhydride. The reaction is driven by the removal of the more volatile acid by distillation. For the more volatile acid anhydrides, such as acetic anhydride, the distillation preferably is carried out at ambient pressure to reach temperatures sufficient to drive the reaction toward completion. For less volatile materials, solvents such as toluene, xylene, glyme, and diglyme could be used with the process to limit temperature. Alternatively, the process could be run at reduced pressure. It would be useful to use up to a twofold excess or more of the acid anhydride which would be distilled out of the mixture after all of the more volatile reaction coproducts, consisting of acids and nonsilane esters, have been distilled out. This excess of acid anhydride would serve to drive the reaction to completion, as well as to help drive the coproducts out of the reaction mixture. At the completion of the reaction, distillation should be continued to drive out the remaining acid anhydride. The product optionally could be distilled.
Reactions 2 and 9 can be carried out in two steps. The first step would involve conversion of the mercaptosilane to a corresponding metal derivative. Alkali metal derivatives, especially sodium or also potassium and lithium, are preferable. The metal derivative would be prepared by adding the alkali metal or a strong base derived from the alkali metal to the mercaptosilane. The reaction would occur at ambient temperature. Appropriate bases would include alkali metal alkoxides, amides, hydrides, and mercaptides. Alkali metal organometallic reagents would also be effective. Grignard reagents would yield magnesium derivatives, which would be another alternative. Solvents, such as toluene, xylene, benzene, aliphatic hydrocarbons, ethers, and alcohols could be used to prepare the alkali metal derivatives. Once the alkali metal derivative is prepared, any alcohol present would need to be removed. This could be done by distillation or evaporation. Alcohols, such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol, and t-butanol may be removed by azeotropic distillation with benzene, toluene, xylene, or aliphatic hydrocarbons. Toluene and xylene are preferable; toluene is most preferable. The second step in the overall process would be to add to this solution, with stirring, the acid chloride or acid anhydride at temperatures between xe2x88x9220xc2x0 C. and the boiling point of the mixture, preferably at temperatures between 0xc2x0 C. and ambient temperature. The product would be isolated by removing the salt and solvent. It could be purified by distillation.
Reactions 3 and 10 could be carried out by distilling a mixture of the mercaptosilane and the ester and optionally a solvent and/or a catalyst. Appropriate boiling temperatures of the mixture would be above 100xc2x0 C. This process leads to a chemical reaction in which the mercapto group of the mercaptosilane is esterified to the thioester silane analog with release of an equivalent of the corresponding alcohol. The reaction is driven by the removal of the alcohol by distillation, either as the more volatile species, or as an azeotrope with the ester. For the more volatile esters the distillation is suitably carried out at ambient pressure to reach temperatures sufficient to drive the reaction toward completion. For less volatile esters, solvents, such as toluene, xylene, glyme, and diglyme could be used with the process to limit temperature. Alternatively, the process could be run at reduced pressure. It is useful to use up to a twofold excess or more of the ester, which would be distilled out of the mixture after all of the alcohol coproduct has been distilled out. This excess ester would serve to drive the reaction to completion as well as to help drive the coproduct alcohol out of the reaction mixture. At the completion of the reaction, distillation would be continued to drive out the remaining ester. The product optionally could be distilled.
Reactions 4 and 11 could be carried out by distilling a mixture of the thioester silane and the other ester and optionally a solvent and/or a catalyst. Appropriate boiling temperatures of the mixture would be above 80xc2x0 C.; preferably above 100xc2x0 C. The temperature would preferably not exceed 250xc2x0 C. This process leads to a chemical reaction in which the thioester group of the thioester silane is transesterified to a new thioester silane with release of an equivalent of a new ester. The new thioester silane generally would be the least volatile species present; however, the new ester would be more volatile than the other reactants. The reaction would be driven by the removal of the new ester by distillation. The distillation can be carried out at ambient pressure to reach temperatures sufficient to drive the reaction toward completion. For systems containing only less volatile materials, solvents, such as toluene, xylene, glyme, and diglyme could be used with the process to limit temperature. Alternatively, the process could be run at reduced pressure. It would be useful to use up to a twofold excess or more of the other ester, which would be distilled out of the mixture after all of the new ester coproduct has been distilled out. This excess other ester would serve to drive the reaction to completion as well as to help drive the coproduct other ester out of the reaction mixture. At the completion of the reaction, distillation would be continued to drive out the remaining said new ester. The product optionally then could be distilled.
Reactions 5 and 12 could be carried out by heating a mixture of the 1-sila-2-thiacyclopentane or the 1-sila-2-thiacyclohexane and the ester with the catalyst. Optionally, the mixture could be heated or refluxed with a solvent, preferably a solvent whose boiling point matches the desired temperature. Optionally a solvent of higher boiling point than the desired reaction temperature can be used at reduced pressure, the pressure being adjusted to bring the boiling point down to the desired reaction temperature. The temperature of the mixture would be in the range of 80xc2x0 to 250xc2x0 C.; preferably 100xc2x0 to 200xc2x0 C. Solvents, such as toluene, xylene, aliphatic hydrocarbons, and diglyme could be used with the process to adjust the temperature. Alternatively, the process could be run under reflux at reduced pressure. The most preferable condition is to heat a mixture of the 1-sila-2-thiacyclopentane or the 1-sila-2-thiacyclohexane and the ester, without solvent, preferably under inert atmosphere, for a period of 20 to 100 hours at a temperature of 120xc2x0 to 170xc2x0 C. using the sodium, potassium, or lithium salt of the acid corresponding to the ester as a catalyst. The process leads to a chemical reaction in which the sulfur-silicon bond of the 1-sila-2-thiacyclopentane or the 1-sila-2-thiacyclohexane is transesterified by addition of the ester across said sulfur-silicon bond. The product is the thioester silane analog of the original 1-sila-2-thiacyclopentane or the 1-sila-2-thiacyclohexane. Optionally, up to a twofold excess or more of the ester would be used to drive the reaction toward completion. At the completion of the reaction, the excess ester can be removed by distillation. The product optionally could be purified by distillation.
Reactions 6 and 13 can be carried out by heating or refluxing a mixture of the alkene-functional silane and the thioacid. Aspects of Reaction 13 have been disclosed previously in U.S. Pat. No. 3,692,812 and by G. A. Gornowicz et al., in J. Org. Chem. (1968), 33(7), 2918-24. The uncatalyzed reaction can occur at temperatures as low as 105xc2x0 C., but often fails. The probability of success increases with temperature and becomes high when the temperature exceeds 160xc2x0 C. The reaction may be made reliable and the reaction brought largely to completion by using UV radiation or a catalyst. With a catalyst, the reaction can be made to occur at temperatures below 90xc2x0 C. Appropriate catalysts are free radical initiators, e.g., peroxides, preferably organic peroxides, and azo compounds. Examples of peroxide initiators include peracids, such as perbenzoic and peracetic acids; esters of peracids; hydroperoxides, such as t-butyl hydroperoxide; peroxides, such as di-t-butyl peroxide; and peroxy-acetals and ketals, such as 1,1-bis(t-butylperoxy)cyclohexane, or any other peroxide. Examples of azo initiators include azobisisobutyronitrile (AIBN), 1,1-azobis(cyclohexanecarbonitrile) (VAZO, DuPont product); and azo-tert-butane. The reaction can be run by heating a mixture of the alkene-functional silane and the thioacid with the catalyst. It is preferable for the overall reaction to be run on an equimolar or near equimolar basis to get the highest conversions. The reaction is sufficiently exothermic that it tends to lead to a rapid temperature increase to reflux followed by a vigorous reflux as the reaction initiates and continues rapidly. This vigorous reaction can lead to hazardous boil-overs for larger quantities. Side reactions, contamination, and loss in yield can result as well from uncontrolled reactions. The reaction can be controlled effectively by adding partial quantities of one reagent to the reaction mixture, initiating the reaction with the catalyst, allowing the reaction to run its course largely to completion, and then adding the remains of the reagent, either as a single addition or as multiple additives. The initial concentrations and rate of addition and number of subsequent additions of the deficient reagent depend on the type and amount of catalyst used, the scale of the reaction, the nature of the starting materials, and the ability of the apparatus to absorb and dissipate heat. A second way of controlling the reaction would involve the continuous addition of one reagent to the other with concomitant continuous addition of catalyst. Whether continuous or sequential addition is used, the catalyst can be added alone and/or preblended with one or both reagents or combinations thereof. Two methods are preferred for reactions involving thiolacetic acid and alkene-functional silanes containing terminal carbonxe2x80x94carbon double bonds. The first involves initially bringing the alkene-functional silane to a temperature of 160xc2x0 to 180xc2x0 C., or to reflux, whichever temperature is lower. The first portion of thiolacetic acid is added at a rate as to maintain up to a vigorous, but controlled, reflux. For alkene-functional silanes with boiling points above 100xc2x0 to 120xc2x0 C., this reflux results largely from the relatively low boiling point of thiolacetic acid (88xc2x0 to 92xc2x0 C., depending on purity) relative to the temperature of the alkene-functional silane. At the completion of the addition, the reflux rate rapidly subsides. It often accelerates again within several minutes, especially if an alkene-functional silane with a boiling point above 120xc2x0 C. is used, as the reaction initiates. If it does not initiate within 10 to 15 minutes, initiation can be brought about by addition of catalyst. The preferred catalyst is di-t-butyl peroxide. The appropriate quantity of catalyst is from 0.2 to 2 percent, preferably from 0.5 to 1 percent, of the total mass of mixture to which the catalyst is added. The reaction typically initiates within a few minutes as evidenced by an increase in reflux rate. The reflux temperature gradually increases as the reaction proceeds. Then the next portion of thiolacetic acid is added, and the aforementioned sequence of steps is repeated. The preferred number of thiolacetic additions for total reaction quantities of about one to about four kilograms is two, with about one-third of the total thiolacetic acid used in the first addition and the remainder in the second. For total quantities in the range of about four to ten kilograms, a total of three thiolacetic additions is preferred, the distribution being approximately 20 percent of the total used in the first addition, approximately 30 percent in the second addition, and the remainder in the third addition. For larger scales involving thiolacetic acid and alkene-functional silanes, it is preferable to use more than a total of three thiolacetic additions and, more preferably, to add the reagents in the reverse order. Initially, the total quantity of thiolacetic acid is brought to reflux. This is followed by continuous addition of the alkene-functional silane to the thiolacetic acid at such a rate as to bring about a smooth but vigorous reaction rate. The catalyst, preferably di-t-butylperoxide, can be added in small portions during the course of the reaction or as a continuous flow. It is best to accelerate the rate of catalyst addition as the reaction proceeds to completion to obtain the highest yields of product for the lowest amount of catalyst required. The total quantity of catalyst used should be 0.5 to 2 percent of the total mass of reagents used. Whichever method is used, the reaction is followed up by a vacuum stripping process to remove volatiles and unreacted thiolacetic acid and silane. The product may be purified by distillation.
Methods to run Reactions 7 and 14 can be carried out in two steps. The first step involves preparation of a salt of the thioacid. Alkali metal derivatives are preferred, with the sodium derivative being most preferred. These salts would be prepared as solutions in solvents in which the salt is appreciably soluble, but suspensions of the salts as solids in solvents in which the salts are only slightly soluble are also a viable option. Alcohols, such as propanol, isopropanol, butanol, isobutanol, and t-butanol, and preferably methanol and ethanol are useful because the alkali metal salts are slightly soluble in them. In cases where the desired product is an alkoxysilane, it is preferable to use an alcohol corresponding to the silane alkoxy group to prevent transesterification at the silicon ester. Alternatively, nonprotic solvents can be used. Examples of appropriate solvents are ethers or polyethers such as glyme, diglyme, and dioxanes; N,N-dimethylformamide; N,N-dimethylacetamide; dimethylsulfoxide; N-methylpyrrolidinone; or hexamethylphosphoramide. Once a solution, suspension, or combination thereof of the salt of the thioacid has been prepared, the second step is to react it with the appropriate haloalkylsilane. This may be accomplished by stirring a mixture of the haloalkylsilane with the solution, suspension, or combination thereof of the salt of the thioacid at temperatures corresponding to the liquid range of the solvent for a period of time sufficient to complete substantially the reaction. Preferable temperatures are those at which the salt is appreciably soluble in the solvent and at which the reaction proceeds at an acceptable rate without excessive side reactions. With reactions starting from chloroalkylsilanes in which the chlorine atom is not allylic or benzylic, preferable temperatures are in the range of 60xc2x0 to 160xc2x0 C. Reaction times can range from one or several hours to several days. For alcohol solvents where the alcohol contains four carbon atoms or fewer, the most preferred temperature is at or near reflux. When diglyme is used as a solvent, the most preferred temperature is in the range of 70xc2x0 to 120xc2x0 C., depending on the thioacid salt used. If the haloalkylsilane is a bromoalkylsilane or a chloroalkylsilane in which the chlorine atom is allylic or benzylic, temperature reductions of 30xc2x0 to 60xc2x0 C. are appropriate relative to those appropriate for nonbenzylic or nonallylic chloroalkylsilanes because of the greater reactivity of the bromo group. Bromoalkylsilanes are preferred over chloroalkylsilanes because of their greater reactivity, lower temperatures required, and greater ease in filtration or centrifugation of the coproduct alkali metal halide. This preference, however, can be overridden by the lower cost of the chloroalkylsilanes, especially for those containing the halogen in the allylic or benzylic position. For reactions between straight chain chloroalkylethoxysilanes and sodium thiocarboxylates to form thiocarboxylate ester ethoxysilanes, it is preferable to use ethanol at reflux for 10 to 20 hours if 5 to 20 percent mercaptosilane is acceptable in the product. Otherwise, diglyme would be an excellent choice, in which the reaction would be run preferably in the range of 80xc2x0 to 120xc2x0 C. for one to three hours. Upon completion of the reaction the salts and solvent should be removed, and the product may be distilled to achieve higher purity.
If the salt of the thioacid to be used in Reactions 7 and 14 is not commercially available, its preparation may be accomplished by one of two methods, described below as Method A and Method B. Method A involves adding the alkali metal or a base derived from the alkali metal to the thioacid. The reaction occurs at ambient temperature. Appropriate bases include alkali metal alkoxides, hydrides, carbonates, and bicarbonates. Solvents, such as toluene, xylene, benzene, aliphatic hydrocarbons, ethers, and alcohols may be used to prepare the alkali metal derivatives. In Method B, acid chlorides or acid anhydrides would be converted directly to the salt of the thioacid by reaction with the alkali metal sulfide or hydrosulfide. Hydrated or partially hydrous alkali metal sulfides or hydrosulfides are available; however, anhydrous or nearly anhydrous alkali metal sulfides or hydrosulfides are preferred. Hydrous materials can be used, however, but with loss in yield and hydrogen sulfide formation as a coproduct. The reaction involves addition of the acid chloride or acid anhydride to the solution or suspension of the alkali metal sulfide and/or hydrosulfide and heating at temperatures ranging from ambient to the reflux temperature of the solvent for a period of time sufficient largely to complete the reaction, as evidenced by the formation of the coproduct salts.
If the alkali metal salt of the thioacid is prepared in such a way that an alcohol is present, either because it was used as a solvent, or because it formed, as for example, by the reaction of a thioacid with an alkali metal alkoxide, it may be desirable to remove the alcohol if a product low in mercaptosilane is desired. In this case, it would be necessary to remove the alcohol prior to reaction of the salt of the thioacid with the haloalkylsilane. This could be done by distillation or evaporation. Alcohols, such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol, and t-butanol are preferably removed by azeotropic distillation with benzene, toluene, xylene, or aliphatic hydrocarbons. Toluene and xylene are preferred.
Utility
The blocked mercaptosilanes described herein are useful as coupling agents for organic polymers (i.e., rubbers) and inorganic fillers. The blocked mercaptosilanes are unique in that the high efficiency of the mercapto group can be utilized without the detrimental side effects typically associated with the use of mercaptosilanes, such as high processing viscosity, less than desirable filler dispersion, premature curing (scorch), and odor. These benefits are accomplished because the mercaptan group initially is nonreactive because of the blocking group. The blocking group substantially prevents the silane from coupling to the organic polymer during the compounding of the rubber. Generally, only the reaction of the silane xe2x80x94SiX3 group with the filler can occur at this stage of the compounding process. Thus, substantial coupling of the filler to the polymer is precluded during mixing, thereby minimizing the undesirable premature curing (scorch) and the associated undesirable increase in viscosity. One can achieve better cured filled rubber properties, such as a balance of high modulus and abrasion resistance, because of the avoidance of premature curing.
In use, one or more of the blocked mercaptosilanes are mixed with the organic polymer before, during, or after the compounding of the filler into the organic polymer. It is preferred to add the silanes before or during the compounding of the filler into the organic polymer, because these silanes facilitate and improve the dispersion of the filler. The total amount of silane present in the resulting combination should be about 0.05 to about 25 parts by weight per hundred parts by weight of organic polymer (phr), more preferably 1 to 10 phr. Fillers can be used in quantities ranging from about 5 to 100 phr, more preferably from 25 to 80 phr.
When reaction of the mixture to couple the filler to the polymer is desired, a deblocking agent is added to the mixture to deblock the blocked mercaptosilane. The deblocking agent may be added at quantities ranging from about 0.1 to about 5 phr, more preferably in the range of from 0.5 to 3 phr. If alcohol or water is present (as is common) in the mixture, a catalyst (e.g., tertiary amines, Lewis acids, or thiols) may be used to initiate and promote the loss of the blocking group by hydrolysis or alcoholysis to liberate the corresponding mercaptosilane. Alternatively, the deblocking agent may be a nucleophile containing a hydrogen atom sufficiently labile such that the hydrogen atom could be transferred to the site of the original blocking group to form the mercaptosilane. Thus, with a blocking group acceptor molecule, an exchange of hydrogen from the nucleophile would occur with the blocking group of the blocked mercaptosilane to form the mercaptosilane and the corresponding derivative of the nucleophile containing the original blocking group. This transfer of the blocking group from the silane to the nucleophile could be driven, for example, by a greater thermodynamic stability of the products (mercaptosilane and nucleophile containing the blocking group) relative to the initial reactants (blocked mercaptosilane and nucleophile). For example, if the nucleophile were an amine containing an Nxe2x80x94H bond, transfer of the blocking group from the blocked mercaptosilane would yield the mercaptosilane and one of several classes of amides corresponding to the type of blocking group used. For example, carboxyl blocking groups deblocked by amines would yield amides, sulfonyl blocking groups deblocked by amines would yield sulfonamides, sulfinyl blocking groups deblocked by amines would yield sulfinamides, phosphonyl blocking groups deblocked by amines would yield phosphonamides, phosphinyl blocking groups deblocked by amines would yield phosphinamides. What is important is that regardless of the blocking group initially present on the blocked mercaptosilane and regardless of the deblocking agent used, the initially substantially inactive (from the standpoint of coupling to the organic polymer) blocked mercaptosilane is substantially converted at the desired point in the rubber compounding procedure to the active mercaptosilane. It is noted that partial amounts of the nucleophile may be used (i.e., a stoichiometric deficiency), if one were to deblock only part of the blocked mercaptosilane to control the degree of vulcanization of a specific formulation.
Water typically is present on the inorganic filler as a hydrate, or bound to a filler in the form of a hydroxyl group. The deblocking agent could be added in the curative package or, alternatively, at any other stage in the compounding process as a single component. Examples of nucleophiles would include any primary or secondary amines, or amines containing Cxe2x95x90N double bonds, such as imines or guanidines, with the proviso that said amine contains at least one Nxe2x80x94H (nitrogen-hydrogen) bond. Numerous specific examples of guanidines, amines, and imines well known in the art, which are useful as components in curatives for rubber, are cited in J. Van Alphen, Rubber Chemicals, (Plastics and Rubber Research Institute TNO, Delft, Holland, 1973). Some examples include N,Nxe2x80x2-diphenylguanidine, N,Nxe2x80x2,Nxe2x80x3-triphenylguanidine, N,Nxe2x80x2-di-ortho-tolylguanidine, orthobiguanide, hexamethylenetetramine, cyclohexylethylamine, dibutylamine, and 4,4xe2x80x2-diaminodiphenylmethane. Any general acid catalysts used to transesterify esters, such as Bronsted or Lewis acids, could be used as catalysts.
The rubber composition need not be, but preferably is, essentially free of functionalized siloxanes, especially those of the type disclosed in Australian Patent AU-A-10082/97, which is incorporated herein by reference. Most preferably, the rubber composition is free of functionalized siloxanes.
In practice, sulfur vulcanized rubber products typically are prepared by thermomechanically mixing rubber and various ingredients in a sequentially stepwise manner followed by shaping and curing the compounded rubber to form a vulcanized product. First, for the aforesaid mixing of the rubber and various ingredients, typically exclusive of sulfur and sulfur vulcanization accelerators (collectively xe2x80x9ccuring agentsxe2x80x9d), the rubber(s) and various rubber compounding ingredients typically are blended in at least one, and often (in the case of silica filled low rolling resistance tires) two, preparatory thermomechanical mixing stage(s) in suitable mixers. Such preparatory mixing is referred to as nonproductive mixing or nonproductive mixing steps or stages. Such preparatory mixing usually is conducted at temperatures up to 140xc2x0 to 200xc2x0 C. and often up to 150xc2x0 to 180xc2x0 C. Subsequent to such preparatory mix stages, in a final mixing stage, sometimes referred to as a productive mix stage, deblocking agent (in the case of this invention), curing agents, and possibly one or more additional ingredients are mixed with the rubber compound or composition, typically at a temperature in a range of 50xc2x0 to 130xc2x0 C., which is a lower temperature than the temperatures utilized in the preparatory mix stages to prevent or retard premature curing of the sulfur curable rubber, which is sometimes referred to as scorching of the rubber composition. The rubber mixture, sometimes referred to as a rubber compound or composition, typically is allowed to cool, sometimes after or during a process intermediate mill mixing, between the aforesaid various mixing steps, for example, to a temperature of about 50xc2x0 C. or lower. When it is desired to mold and to cure the rubber, the rubber is placed into the appropriate mold at about at least 130xc2x0 C. and up to about 200xc2x0 C., which will cause the vulcanization of the rubber by the mercapto groups on the mercaptosilane and any other free sulfur sources in the rubber mixture.
By thermomechanical mixing, it is meant that the rubber compound, or composition of rubber and rubber compounding ingredients, is mixed in a rubber mixture under high shear conditions where it autogenously heats up as a result of the mixing primarily due to shear and associated friction within the rubber mixture in the rubber mixer. Several chemical reactions may occur at various steps in the mixing and curing processes.
The first reaction is a relatively fast reaction and is considered herein to take place between the filler and the SiX3 group of the blocked mercaptosilane. Such reaction may occur at a relatively low temperature such as, for example, at about 120xc2x0 C. The second and third reactions are considered herein to be the deblocking of the mercaptosilane and the reaction which takes place between the sulfuric part of the organosilane (after deblocking), and the sulfur vulcanizable rubber at a higher temperature, for example, above about 140xc2x0 C.
Another sulfur source may be used, for example, in the form of elemental sulfur as S8. A sulfur donor is considered herein as a sulfur containing compound which liberates free, or elemental, sulfur at a temperature in a range of 140xc2x0 to 190xc2x0 C. Examples of such sulfur donors may be, but are not limited to, polysulfide vulcanization accelerators and organosilane polysulfides with at least two connecting sulfur atoms in its polysulfide bridge. The amount of free sulfur source addition to the mixture can be controlled or manipulated as a matter of choice relatively independently from the addition of the aforesaid blocked mercaptosilane. Thus, for example, the independent addition of a sulfur source may be manipulated by the amount of addition thereof and by sequence of addition relative to addition of other ingredients to the rubber mixture.
Addition of an alkyl silane to the coupling agent system (blocked mercaptosilane plus additional free sulfur source and/or vulcanization accelerator) typically in a mole ratio of alkyl silane to blocked mercaptosilane in a range of 1/50 to 1/2 promotes an even better control of rubber composition processing and aging.
A rubber composition is prepared by a process which comprises the sequential steps of:
(A) thermomechanically mixing, in at least one preparatory mixing step, to a temperature of 140xc2x0 to 200xc2x0 C., alternatively to 140xc2x0 to 190xc2x0 C., for a total mixing time of 2 to 20 minutes, alternatively 4 to 15 minutes, for such mixing step(s);
(i) 100 parts by weight of at least one sulfur vulcanizable rubber selected from conjugated diene homopolymers and copolymers, and copolymers of at least one conjugated diene and aromatic vinyl compound,
(ii) 5 to 100 phr (parts per hundred rubber), preferably 25 to 80 phr, of particulate filler, wherein preferably the filler contains 1 to 85 weight percent carbon black,
(iii) 0.05 to 20 parts by weight filler of at least one blocked mercaptosilane;
(B) subsequently blending therewith, in a final thermomechanical mixing step at a temperature to 50xc2x0 to 130xc2x0 C. for a time sufficient to blend the rubber, preferably between 1 to 30 minutes, more preferably 1 to 3 minutes, at least one deblocking agent at about 0.05 to 20 parts by weight of the filler and a curing agent at 0 to 5 phr; and optionally
(C) curing said mixture at a temperature of 130xc2x0 to 200xc2x0 C. for about 5 to 60 minutes.
The process may also comprise the additional steps of preparing an assembly of a tire or sulfur vulcanizable rubber with a tread comprised of the rubber composition prepared according to this invention and vulcanizing the assembly at a temperature in a range of 130xc2x0 to 200xc2x0 C.
Suitable organic polymers and fillers are well known in the art and are described in numerous texts, of which two examples include The Vanderbilt Rubber Handbook, R. F. Ohm, ed. (R. T. Vanderbilt Company, Inc., Norwalk, Conn., 1990), and Manual for the Rubber Industry, T. Kempermann, S. Koch, and J. Sumner, eds. (Bayer AG, Leverkusen, Germany, 1993). Representative examples of suitable polymers include solution styrene-butadiene rubber (sSBR), styrene-butadiene rubber (SBR), natural rubber (NR), polybutadiene (BR), ethylene-propylene co- and ter-polymers (EP, EPDM), and acrylonitrile-butadiene rubber (NBR). The rubber composition is comprised of at least one diene-based elastomer, or rubber. Suitable conjugated dienes are isoprene and 1,3-butadiene and suitable vinyl aromatic compounds are styrene and alpha methyl styrene. Thus, the rubber is a sulfur curable rubber. Such diene based elastomer, or rubber, may be selected, for example, from at least one of cis-1,4-polyisoprene rubber (natural and/or synthetic, and preferably natural rubber), emulsion polymerization prepared styrene/butadiene copolymer rubber, organic solution polymerization prepared styrene/butadiene rubber, 3,4-polyisoprene rubber, isoprene/butadiene rubber, styrene/isoprene/butadiene terpolymer rubber, cis-1,4-polybutadiene, medium vinyl polybutadiene rubber (35 percent to 50 percent vinyl), high vinyl polybutadiene rubber (50 percent to 75 percent vinyl), styrene/isoprene copolymers, emulsion polymerization prepared styrene/butadiene/acrylonitrile terpolymer rubber and butadiene/acrylonitrile copolymer rubber. An emulsion polymerization derived styrene/butadiene (eSBR) might be used having a relatively conventional styrene content of 20 percent to 28 percent bound styrene or, for some applications, an eSBR having a medium to relatively high bound styrene content, namely, a bound styrene content of 30 percent to 45 percent. Emulsion polymerization prepared styrene/butadiene/acrylonitrile terpolymer rubbers containing 2 to 40 weight percent bound acrylonitrile in the terpolymer are also contemplated as diene based rubbers for use in this invention.
The solution polymerization prepared SBR (sSBR) typically has a bound styrene content in a range of 5 to 50 percent, preferably 9 to 36 percent. Polybutadiene elastomer may be conveniently characterized, for example, by having at least a 90 weight percent cis-1,4-content.
Representative examples of suitable filler materials include metal oxides, such as silica (pyrogenic and precipitated), titanium dioxide, aluminosilicate and alumina, siliceous materials including clays and talc, and carbon black. Particulate, precipitated silica is also sometimes used for such purpose, particularly when the silica is used in connection with a silane. In some cases, a combination of silica and carbon black is utilized for reinforcing fillers for various rubber products, including treads for tires. Alumina can be used either alone or in combination with silica. The term xe2x80x9caluminaxe2x80x9d can be described herein as aluminum oxide, or Al2O3. The fillers may be hydrated or in anhydrous form. Use of alumina in rubber compositions can be shown, for example, in U.S. Pat. No. 5,116,886 and EP 631,982.
The blocked mercaptosilane may be premixed, or prereacted, with the filler particles or added to the rubber mix during the rubber and filler processing, or mixing stage. If the silane and filler are added separately to the rubber mix during the rubber and filler mixing, or processing stage, it is considered that the blocked mercaptosilane then combines in situ with the filler.
The vulcanized rubber composition should contain a sufficient amount of filler to contribute a reasonably high modulus and high resistance to tear. The combined weight of the filler may be as low as about 5 to 100 phr, but is more preferably from 25 phr to 85 phr.
Precipitated silicas are preferred as the filler. The silica may be characterized by having a BET surface area, as measured using nitrogen gas, preferably in the range of 40 to 600 m2/g, and more usually in a range of 50 to 300 m2/g. The silica typically may also be characterized by having a dibutylphthalate (DBP) absorption value in a range of 100 to 350, and more usually 150 to 300. Further, the silica, as well as the aforesaid alumina and aluminosilicate, may be expected to have a CTAB surface area in a range of 100 to 220. The CTAB surface area is the external surface area as evaluated by cetyl trimethylammonium bromide with a pH of 9. The method is described in ASTM D 3849.
Mercury porosity surface area is the specific surface area determined by mercury porosimetry. For such technique, mercury is penetrated into the pores of the sample after a thermal treatment to remove volatiles. Set up conditions may be suitably described as using a 100 mg sample, removing volatiles during two hours at 105xc2x0 C. and ambient atmospheric pressure, ambient to 2000 bars pressure measuring range. Such evaluation may be performed according to the method described in Winslow, Shapiro in ASTM bulletin, page 39 (1959) or according to DIN 66133. For such an evaluation, a CARLO-ERBA Porosimeter 2000 might be used. The average mercury porosity specific surface area for the silica should be in a range of 100 to 300 m2/g.
A suitable pore size distribution for the silica, alumina, and aluminosilicate according to such mercury porosity evaluation is considered herein to be:
5 percent or less of its pores have a diameter of less than about 10 nm; 60 percent to 90 percent of its pores have a diameter of 10 to 100 nm; 10 percent to 30 percent of its pores have a diameter of 100 to 1,000 nm; and 5 percent to 20 percent of its pores have a diameter of greater than about 1,000 nm.
The silica might be expected to have an average ultimate particle size, for example, in the range of 0.01 to 0.05 xcexcm as determined by the electron microscope, although the silica particles may be even smaller, or possibly larger, in size. Various commercially available silicas may be considered for use in this invention such as, from PPG Industries under the HI-SIL trademark with designations HI-SIL 210, 243, etc.; silicas available from Rhone-Poulenc, with, for example, designation of ZEOSIL 1165MP; silicas available from Degussa with, for example, designations VN2 and VN3, etc.; and silicas commercially available from Huber having, for example, a designation of HUBERSIL 8745.
Where it is desired for the rubber composition, which contains both a siliceous filler such as silica, alumina and/or aluminosilicates and also carbon black reinforcing pigments, to be primarily reinforced with silica as the reinforcing pigment, it is often preferable that the weight ratio of such siliceous fillers to carbon black is at least 3/1 and preferably at least 10/1 and, thus, in a range of 3/1 to 30/1. The filler may be comprised of 15 to 95 weight percent precipitated silica, alumina, and/or aluminosilicate and, correspondingly 5 to 85 weight percent carbon black, wherein the carbon black has a CTAB value in a range of 80 to 150. Alternatively, the filler can be comprised of 60 to 95 weight percent of said silica, alumina, and/or aluminosilicate and, correspondingly, 40 to 5 weight percent carbon black. The siliceous filler and carbon black may be preblended or blended together in the manufacture of the vulcanized rubber.
The rubber composition may be compounded by methods known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials. Examples of such commonly used additive materials include curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, silicas, plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agents, and reinforcing materials, such as, for example, carbon black. Depending on the intended use of the sulfur vulcanizable and sulfur vulcanized material (rubbers), the additives mentioned above are selected and commonly used in conventional amounts.
The vulcanization may be conducted in the presence of an additional sulfur vulcanizing agent. Examples of suitable sulfur vulcanizing agents include, for example, elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amino disulfide, polymeric polysulfide, or sulfur olefin adducts which are conventionally added in the final, productive, rubber composition mixing step. The sulfur vulcanizing agents (which are common in the art) are used, or added in the productive mixing stage, in an amount ranging from 0.4 to 3 phr, or even, in some circumstances, up to about 8 phr, with a range of from 1.5 to 2.5 phr, sometimes from 2 to 2.5 phr, being preferred.
Vulcanization accelerators, i.e., additional sulfur donors, may be used herein. It is appreciated that they may be, for example, of the type such as, for example, benzothiazole, alkyl thiuram disulfide, guanidine derivatives, and thiocarbamates. Representative of such accelerators are, for example, but are not limited to, mercapto benzothiazole, tetramethyl thiuram disulfide, benzothiazole disulfide, diphenylguanidine, zinc dithiocarbamate, alkylphenoldisulfide, zinc butyl xanthate, N-dicyclohexyl-2-benzothiazolesulfenamide, N-cyclohexyl-2-benzothiazolesulfenamide, N-oxydiethylenebenzothiazole-2-sulfenamide, N,N-diphenylthiourea, dithiocarbamylsulfenamide, N,N-diisopropylbenzothiozole-2-sulfenamide, zinc-2-mercaptotoluimidazole, dithiobis(N-methyl piperazine), dithiobis(N-beta-hydroxy ethyl piperazine) and dithiobis(dibenzyl amine). Other additional sulfur donors, may be, for example, thiuram and morpholine derivatives. Representative of such donors are, for example, but not limited to, dimorpholine disulfide, dimorpholine tetrasulfide, tetramethyl thiuram tetrasulfide, benzothiazyl-2,N-dithiomorpholide, thioplasts, dipentamethylenethiuram hexasulfide, and disulfidecaprolactam.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., a primary accelerator. Conventionally and preferably, a primary accelerator(s) is used in total amounts ranging from 0.5 to 4 phr, preferably 0.8 to 1.5 phr. Combinations of a primary and a secondary accelerator might be used with the secondary accelerator being used in smaller amounts (of 0.05 to 3 phr) in order to activate and to improve the properties of the vulcanizate. Delayed action accelerators may be used. Vulcanization retarders might also be used. Suitable types of accelerators are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates, and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate, or thiuram compound.
Typical amounts of tackifier resins, if used, comprise 0.5 to 10 phr, usually 1 to 5 phr. Typical amounts of processing aids comprise 1 to 50 phr. Such processing aids include, for example, aromatic, naphthenic, and/or paraffinic processing oils. Typical amounts of antioxidants comprise 1 to 5 phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others such as those disclosed in the Vanderbilt Rubber Handbook (1978), pages 344-46. Typical amounts of antiozonants comprise 1 to 5 phr. Typical amounts of fatty acids, which, if used, can include stearic acid, comprise 0.5 to 3 phr. Typical amounts of zinc oxide comprise 2 to 5 phr. Typical amounts of waxes comprise 1 to 5 phr. Often microcrystalline waxes are used. Typical amounts of peptizers comprise 0.1 to 1 phr. Typical peptizers may be, for example, pentachlorothiophenol and dibenzamidodiphenyl disulfide.
The rubber composition of this invention can be used for various purposes. For example, it can be used for various tire compounds. Such tires can be built, shaped, molded, and cured by various methods which are known and will be readily apparent to those having skill in such art.
All references cited are incorporated herein as they are relevant to the present invention.